In the collaborative effort towards standardisation of out-of-plane permeability measurement, an international benchmarking exercise was carried out whereby 19 participants worldwide were instructed to measure the out-of-plane permeability following a number of strict guidelines, informed by the outcomes of the first international benchmarking exercise completed in 2021. This paper presents the results of the exercise and an assessment of the reproducibility of the data and the suitability of the proposed test method. The data returned were subjected to a number of statistical analysis methods, which showed that adherence to the test guidelines resulted in a high likelihood of a participant not being an outlier and therefore providing evidence that the test method proposed in this paper is a suitable way forward for a standardised test method.@en

Thin-ply carbon fiber reinforced polymers (CFRP) have claimed significant attention for their potential to surpass traditional composite materials in terms of performance metrics such as first-ply damage criteria, fatigue life, and ultimate strength. This study focuses on investigating the friction behavior of dry carbon fiber tow during mechanical bar spreading, a crucial process in the manufacturing of thinply CFRP. By systematically examining the interplay of wrap angle, tow pre-tension, and final tension, insights are provided into the frictional forces exerted on the carbon fibers. The study utilizes an experimental framework to analyze single-bar and multi-bar setups, considering both symmetric and asymmetric configurations. Results reveal non-linear friction behavior, with increasing wrap angles leading to decreased dynamic friction coefficients. Additionally, results seem to suggest that higher pretension reduces internal tow movement, thereby decreasing friction losses. Multi-bar setups exhibit distinct friction profiles compared to single-bar setups, especially for larger wrap angles and asymmetric cases, indicating the influence of superimposed wrap angles on friction. Recommendations for future research include further exploration of factors such as non-uniform normal loads and relaxation distances between spreader bars to enhance modeling accuracy and optimize friction performance.@en

The transverse permeability of roving/tow-based fiber reinforcement is of great importance for accurate flow modeling in the pultrusion process. This study proposes an experimental approach to characterize the roving-based fiber beds' permeability under different compaction conditions. The experimental permeability results of thick roving-based preforms were reported and compared with the permeability values of roving-based preforms in the literature. A representative preform was infused under vacuum conditions. Its thickness was varied to replicate the different compaction values observed in permeability tests. Micrographs were then collected from it and analyzed to highlight the microscale transformations caused by processing/compaction on the fiber arrangement. The analysis revealed that compaction resulted in the reorganization of filaments along the direction of the applied compaction. Overall, the uniformity of the spatial filament distribution, i.e., the homogeneity within the fibrous domain, increased with increasing compaction. Furthermore, the microstructural analysis demonstrated transverse anisotropy within the tested domains, indicating that the obtained permeability results represented an upper boundary. In addition to the experimental analyses, various transverse permeability models, which were developed based on recently introduced statistical descriptors of fiber distribution, were evaluated by using the statistical descriptors extracted from the analyzed cross-sections. Among these models, the one correlating the second neighbor fiber distance with apparent permeability exhibited good agreement with the experimental results. Highlights: Transverse permeability measurement of a roving-based reinforcement was presented. The influence of compaction on the microstructure was investigated at the filament level. Filament distribution in a pultruded profile was analyzed by using statistical descriptors. The results of the experiments and the models in the literature were compared. The correlation between microstructural features and apparent permeability was discussed.

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This paper presents an experimental investigation into the compaction behavior of roving-based unidirectional (UD) reinforcements. Understanding the complex dynamics of roving compaction is essential for ensuring the quality in composite manufacturing processes and in return to reach optimal performance. In continuous manufacturing processes like pultrusion or tape manufacturing, achieving proper compaction of rovings is crucial for reaching high fiber volume fractions and consequently, the desired mechanical properties in the final composite profile. However, high compaction in rovings poses challenges, particularly during the impregnation phase, where pressure buildup can occur, potentially leading to the formation of undesirable voids or to excessive forces on fiber, causing fuzz accumulation. Balancing the need for high compaction with the risk of defects is thus a delicate exercise, critical for ensuring the quality and integrity of the final product. Through experimental characterization and analysis, this work examines the influence of various factors to the compressibility of rovings, including the application of tension and different configurations in dry conditions. This study aims to provide valuable insights into characterizing the compaction behavior of roving-based reinforcements and the effect of applied tension, thereby advancing the state-of-the-art in continuous composite manufacturing processes.@en

This study presents the first steps of a benchmarking exercise on image processing of composite materials. Employing three distinct imaging protocols and five different image processing algorithms, the research explains the variability in capturing microstructural features of common micrograph dataset. Results highlight the sensitivity of different methods to factors like illumination inhomogeneities and pixel density, influencing the accuracy and consistency of obtained results. By comparing methodologies from different researchers in a blind format, the study identifies strengths and limitations, laying the groundwork for future benchmarking activities. Moving forward, this research sets the stage for standardized protocols and guidelines, aiming to enhance the reproducibility and reliability of microstructural analysis in composite materials. Such efforts are crucial for advancing material design and development, ultimately creating tailored composite materials with enhanced performance and functionality.@en

Transverse permeability is a key parameter to construct accurate process modelling frameworks for composite manufacturing processes whereby dry reinforcements are impregnated by a resin. The transverse permeability of fibrous structures is strongly correlated with the spatial fiber distribution which is inevitably irregular through the cross section. This irregularity and possible resin channel formation through the cross section cause complex permeability field. In this study, we analysed the correlation between the microstructural features and the apparent transverse permeability. Multiple regions of interest were collected from a pultruded part’s cross section that exhibited apparent resin rich layers. Apparent transverse permeability values of these Regions of Interest (ROIs) were calculated by using OpenFOAM. The principal components of the transverse were calculated, and the statistical descriptors were captured for the corresponding ROIs. The principal permeability component analysis showed that principal directions are in-line with the visual predictions. The correlation between the permeability components and the statistical descriptors shows some traces of the capability of predicting the transverse permeability and the principal components based on the microstructural features. A larger dataset and more elaborate statistical descriptors are needed for accurate prediction of the flow behaviour and the apparent permeability components.@en

Thin ply composites open up new opportunities to exceed the limits of conventional composite materials by improving first-ply/ first-damage criteria, fatigue life and ultimate strength [1]. By definition, individual plies with a ply thickness of less than 0.100 mm can be called a thin ply [2,3]. The size effects [1] and design freedom as it allows smaller pitch angles for a specific thickness [4] make thin ply composites superior to conventional ply composites. Obtaining thin plies is possible spreading conventional tows via techniques based on airflow, ultrasonic vibration, and mechanical means as the common alternatives [5]. Among these, mechanical tow spreading is based on pulling dry tows through bars/ pins with a certain tension. An example of a lab-scale tow spreading line, developed at TU Delft, including static spreader bars and tension sensors can be seen in Fig. 1 (a). Wrap angle, the pre-tension of the tow, relative friction between the spreading bar and tow, temperature and pulling speed are some of the parameters influential on spreading. Therefore, we propose an experimental framework to assess and quantify the effect of individual mechanisms on tow spreading. In the present study, we first investigate the effect of tension and wrap angle on tow spreading under the static condition schematically shown in Fig. 1 (b). Then, the effect of an actively controlled bar on tow spreading is tested under static conditions. Spreading bar rotation is controlled with a motor that reveals the influence of relative motion between the bar and the tow and thus the induced friction. Tow spreading is a continuous process, where filaments’ reorganization is time dependent. That means the time spent on a spreading bar, defined by the pulling speed, is influential on spreading, noting that the pulling speed is also intrinsically related to friction. Thus the third experimental method is based on observing tow spreading while the tow is pulled with various pulling rates, as schematically shown in Fig. 1 (d). During the talk, we will report our findings regarding the extent of spreading of different fiber types under different process conditions obtained by controlling the wrap angle, relative speed between the tow and bar(s) as well as by using bars that either are heated and/or exhibits different surface roughness. The analyses will be further enriched by microstructural analyses of specimens. @en

Over the last 70 years, pultrusion has matured into an industry-leading process when it comes to providing high throughput and automated composite manufacture at a competitive price point. In this paper, we review recent innovations that have advanced pultrusion to a versatile manufacturing technology and thereby allowed composite materials to penetrate markets in, e.g., the automotive, construction, aerospace, and wind turbine industries. We accompany our review with discussions on how pultrusion has enabled new innovations within additive manufacturing and sustainable composite manufacturing, and finally, we provide an outlook and suggestions for where we see the potential for research and new industrial applications of pultrusion technology.

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This study presents a detailed material characterization study of a pultrusion specific polyurethane resin system. Firstly, the chemical behaviour was characterized by utilizing differential scanning calorimetry. The cure kinetics was fitted well to an autocatalytic cure kinetics model with Arrhenius temperature dependency. The resin system did not show any inhibition time. Then, the viscosity evolution was observed using a rheometer. From these measurements, the gelation point was estimated from the criterion of cross-over point of storage and loss modulus curves. The corresponding cure degree of gelation point was found to be 0.79 using the cure kinetics model. The complex viscosity evolution through dynamic scans was predicted well with a cure degree and temperature dependent viscosity model. The elastic modulus and glass transition temperature of fully and partially cured samples were measured by means of DMA in tension mode. A five step cure hardening instantaneous linear elastic model was fitted well to a wide range of cure degree values after gelation. Lastly, the fitted material models were employed in a case study of a typical pultrusion process to visualize the effects of pulling speed on the material property evolution.

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Process-induced stress and deformation are critical factors when ensuring product quality and structural integrity of composite profiles manufactured using thermoset pultrusion processes. In this paper, we present a new steady-state 3D-Eulerian numerical framework that enables 9–35 times faster computations compared to the current state-of-the-art quasi-static 3D-methods. In addition, we show how process-induced effects from the profile-advancing pulling force and an initial compressive stress state can be modelled. We demonstrate in theoretical parameter studies that the pulling force advances die-detachment and reduces die-swelling, while the initial compressive stress state has the opposite but a more pronounced effect.

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Pultruded fiber-reinforced polymer composites are susceptible to microstructural nonuni-formity such as variability in fiber volume fraction (V_f ), which can have a profound effect on process-induced residual stress. Until now, this effect of non-uniform V_f distribution has been hardly addressed in the process models. In the present study, we characterized the V_f distribution and accompanying nonuniformity in a unidirectional fiber-reinforced pultruded profile using optical light microscopy. The identified nonuniformity in V_f was subsequently implemented in a mesoscale thermal–chemical–mechanical process model, developed explicitly for the pultrusion process. In our process model, the constitutive material behavior was defined locally with respect to the corresponding fiber volume fraction value in different-sized representative volume elements. The effect of nonuniformity on the temperature and cure degree evolution, and residual stress was analyzed in depth. The results show that the nonuniformity in fiber volume fraction across the cross-section increased the absolute magnitude of the predicted residual stress, leading to a more scattered residual stress distribution. The observed V_f gradient promotes tensile residual stress at the core and compressive residual stress at the outer regions. Consequently, it is concluded that it is essential to take the effects of nonuniformity in fiber distribution into account for residual stress estimations, and the proposed numerical framework was found to be an efficient tool to study this aspect.

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This paper concerns non-isothermal flow in a thermoset resin-injection pultrusion process. Supported by temperature measurements from an industrial pultrusion line and a material characterisation study (curing kinetics, chemorheology, and permeability), the material flow was analysed for the manufacture of a thick glass-fibre profile saturated with a pultrusion-specific polyurethane resin. A central finding is that the heating configuration, together with the strongly convective flow near inlets resulted in phase transitions that were both concave and convex-shaped. This is different from existing literature that commonly describes curing being initiated from die-walls, resulting in the concave phase-transitions.

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The exothermic reaction and overheating during radical polymerization of an Elium® resin and glass fiber reinforced Elium® composites are critically evaluated in this work. The polymerization kinetics of the Elium® resin is obtained by performing differential scanning calorimetry (DSC) scans. The measured data is fit to a temperature and degree of polymerization (DoP) dependent kinetics model. A coupled thermo-chemical process model is developed to predict the temperature evolution and DoP. First, the model is validated with the water bath experiments in which the pure Elium® resin is polymerized at different temperatures (30, 50 and 70 °C). The validated process model is then applied to a vacuum infusion process of glass reinforced Elium® composite laminates with different thicknesses (3.8, 7.5 and 11.3 mm) at room temperature. The produced laminates have the void content lower than 1%. The peak temperature is found to be approximately in the range of 155-160 °C during the water bath experiments. On the other hand, the peak exothermic temperature is approximately 49 °C and 70 °C for 3.8 mm and 11.3 mm thick laminate, respectively. The developed polymerization kinetics model is found to be effective as the predicted temperature evolutions match well with the measured temperatures for different laminates. The effect of laminate thickness and processing teperature on the peak temperature is studied by the developed numerical model. The thermo-chemical process model developed in this work is therefore capable of predicting the polymerization overheating for Elium® composites and can enable the optimization of the manufacturing process to control the thermal and DoP histories.

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The resin injection pultrusion is an automated composite manufacturing method in which the resin is injected in a chamber. The flow and the thermo chemical mechanical (TCM) models have been studied for the pultrusion process to improve the reliability of the final products. Flow models are needed to understand and describe the fiber impregnation, filling time and presence of dry spots or voids. Also pressure field in the injection chamber can be estimated with flow models. TCM models are needed to predict residual stress distributions and to optimize the process conditions. A non-uniform fiber distribution strongly affects the results of both types of models. In this study, different strategies are carried out to implement non-uniform fiber distributions into the models. The cross-sectional image and fiber distribution of a 19×19 mm glass fiber reinforced polyester unidirectional pultruded composite is used. Non-uniform fiber distribution is observed and implemented into the flow model by means of permeability variations. The results of this study are compared with uniform fiber distribution results. In the TCM model, the non-uniform fiber volume content is implemented within different sized patches. The results show that the non-uniform fiber fraction should be taken into account for the process models of composites in order to capture the local process induced stresses and probability of dry spots or voids due to poor fiber impregnation.

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Process induced residual stresses are one of the main sources of defects such as (pre) mature matrix cracking during pultrusion of fiber reinforced polymer composite profiles. Recently, comprehensive process models have been developed to understand and describe the underlying mechanisms of the residual stresses in pultrusion processes. The predicted stresses however have not been validated with experimental measurements which are necessary to verify the implemented material models and assumptions in the process models. A hole drilling method with digital image correlation (DIC) is used in the present work to measure the transverse strain relaxation due to material removal in a pultruded thick composite profile (20 × 20 mm) made of unidirectional glass/polyester. The corresponding residual stress state is back calculated using the measured strains in a finite element based numerical model. The estimated level of transverse residual stress in the core of the profile was found to be 6.1 MPa in tension.@en

Process induced stresses inherently exist in fiber reinforced polymer composites particularly in thick parts due to the presence of non-uniform cure, shrinkage and thermal expansion/contraction during manufacturing. In order to increase the reliability and the performance of the composite materials, process models are developed to predict the residual stress formation. The accuracy of the process models is dependent on the geometrical (micro to macro), material and process parameters as well as the numerical implementation. Therefore, in order to have reliable process modelling framework, there is a need for validation and if necessary calibration of the developed models. This study focuses on measurement of the transverse residual stresses in a relatively thick pultruded profile (20×20 mm) made of glass/polyester. Process-induced residual stresses in the middle of the profile are examined with different techniques which have never been applied for transverse residual stresses in thick unidirectional composites. Hole drilling method with strain gage and digital image correlation are employed. Strain values measured from measurements are used in a finite element model (FEM) to simulate the hole drilling process and predict the residual stress level. The measured released strain is found to be approximately 180 μm/m from the strain gage. The tensile residual stress at the core of the profile is estimated approximately as 7-10 MPa. Proposed methods and measured values in this study will enable validation and calibration of the process models based on the residual stresses.

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A numerical model is developed to predict the process induced deformations of continuous fibre reinforced plastic plates which is made of AS4/8552 (Carbon fibre/epoxy) prepregs. In this way, Manufacturing costs can be reduced with elimination of trial and error approach. Sources of the process induced residual stress are clarified with the help of literature and implemented in 3D numerical model. In addition to the literature review to examine separate sources for residual stresses, an experimental measurement is conducted to observe tool part interaction which is quantified with strain gage which is embedded in prepreg. Numerical model is modified and verified with respect to overall deformation field of manufactured parts. Individual contribution of different mechanisms on overall deformation is discussed with the help of the numerical model. Moreover anisotropic friction between tool and the part is proposed to enhance the predictions of 3-D numerical model.@en